Years ago one of the Ham magazines did a write up on some of the more unusual items that Unca' Sam had on his big olive drab test bench. The BC-221 was one of them... Not because of the design sophistication or exceptional parts quality, but because each meter had an extensive calibration procedure.
IIRC, each unit coming off the line was tuned to a long list of calibration frequencies. The knob settings for each frequency was recorded on a typed reference card specific for that meter, possibly in duplicate. The field operator would tune an unknown signal then compare the knob settings against the calibration card. This was a very time consuming manufacturing process and given the large number of BC-221's on the surplus market the question became how-in-the-hell were they able to calibrate that many meters before the war ended?
It's one thing to crank out combat boots in quantity, but a boot doesn't need to be documented in detail........
I posted the URL as this particular example appears to have aged gracefully and thanks to the fungus proofing has historic provenance
. For all I know the cal card is also dated, but one glance tells you it's the real deal from 1944. Which is why Vinnie wants it.